1. The problem statement, all variables and given/known data f(x) = cos^2(x) - 2sin(x) 0≤x≤2∏ 2. Relevant equations 3. The attempt at a solution f'(x)=-2[(cosX)(sinX) + cosX] f''(x) = -2[ (cos^2(x)-sin^2(x)-sinX] I know there's an identity there for cos2x but it doesn't seem to help me. I also tried to go the other way and use the pythagorean identity and got 2sin^2(x)-sinX=1 but none of these make it easy to tell where f''(x)=0 Further, the book says something like if f''(c)=0 you get no information about concavity. What do they mean? Can't you simply change your c to another number until you get something that's greater or less than 0?